Development of GnRH antagonists has been slow because of low potency of early prototype analogs and histamine-like side-effects caused by more potent analogs. Newer antagonists, free of histamine-like adverse reactions, are being developed like the RS-26306. Twenty-two men were given increasing doses, from 0.03 to 12 mg, of this RS-26306 antagonist as single injections. Testosterone levels decreased significantly for 24 h after the 1 mg dose, and remained low for more than 44 h after doses of 3,6, and 12 mg.